Transmission electron microscopy (TEM) is a technique that employs a beam of electrons to ascertain details at the atomic level of a sample of interest. Such samples can vary greatly in their physical properties, ranging from ductile metals to brittle ceramics.
The preparation of a sample for TEM analysis is typically an arduous, non-routine process because of the requirement that the sample be transparent to electrons. Thus, the sample must be extremely thin, less than 1 micrometer, preferably less than 500 nanometers, more preferably less than 100 nanometers. Consequently, a large part of TEM sample preparation has to do with the removal of unwanted material.
Transmission electron microscopes in current use typically require samples in the form of electron-transparent discs having a diameter of 3 millimeters. Various methods have been developed for constructing individual extremely thin discs; such methods include slicing, sawing, grinding, electropolishing, and ion milling. Techniques for TEM sample preparation are discussed in, for example, Williams and Carter, Transmission Electron Microscopy: A Textbook for Materials Science, 1996, Plenum Press, New York, pages 3-17 and 155-173, and Rose and Flutie, "Sample Preparation for Transmission Electron Microscopy Studies in Microelectronics" in Microelectronics Failure Analysis, 1990, ASM International, Metals Park, Ohio, pages 13-1 to 13-7, the disclosures of which are incorporated herein by reference.
Reducing the effort of TEM sample preparation has been a goal of many workers in the field. Varker and Chang, "Preparation of Large-area, Electron-Transparent Silicon Specimens by Anisotropic Etching" in Solid State Technology, April 1983, pages 143-146, describe thinning 48 millimeter windows of electron-transparent silicon over an entire wafer. However the resultant wafer is too fragile for further processing. Other workers have proposed depositing a thin film directly on TEM grids coated with a carbon support film. The deposited film is, however, not representative of the manufacturing process. Furthermore there is an unacceptable risk of contamination to the sample film and deposition equipment.
Thus, there remains a need for a convenient preparation of samples for TEM analysis that can be applied to a wide variety of materials and enables the simultaneous preparation of multiple samples. The present invention meets this need.